Abstract: An installation for forming a tube of thermoplastic material with a socket having an external bead thereon, by pushing the tube end on a mandril consisting of two, spaced apart, axially movable mandril parts, and moving the two mandril parts towards each other while the outer surface of the tube is gripped by a mould having an internal recess corresponding to the bead to be formed on the tube. Compressed air, a compressible expandable rubber body, die or an inflatable rubber body is present in the space bounded by the two mandril parts and the inner surface of the tube. The length of the tube between the two mandril parts in their initial position is at least equal to the length of the bead to be formed.